1. Field of the Invention
This invention relates to the preparation of glasses from batch materials, and is particularly concerned with the preparation of high purity glasses such as low optical loss glasses required for optical fibre manufacture and those required as host glasses for laser manufacture.
2. Description of the Prior Art
When glasses are prepared in the normal way by heating in an electric or flame furnace, the glass is subject to contamination from three main sources, that from the hot refractory lining of the furnace, that from the heating element or flame, and that from the crucible containing the melt. Two of the factors affecting the amount of contamination are temperature and duration. Hence the contamination which occurs during the initial heating required to cause the batch materials to coalesce is normally less than that which occurs during the longer higher temperature heating required for fining and homogenising the glass. A description of a known system for using radio frequency to prepare pure glass and cooling of crucibles with gas to reduce contamination is found in copending application Ser. No. 479,004, filed June 13, 1974 and assigned to the same assignee as the instant application.
At room temperature the a.c. conductivities of most glasses, and also of their batch constituents, are so low that r.f. inductive heating from room temperature is impractical and hence an alternative method of heating is used to heat the charge to a temperature at which it readily couples into the applied field. With some glasses the conductivity, even in the molten state, is so low that it is preferable to use r.f. dielectric heating and make use instead of dielectric losses. The choice of an appropriate frequency, which may typically lie in the range from one to several thousand MHz will depend upon the electrical properties of the particular glass composition being prepared. Thus the manner of application of the r.f. power will depend upon the absolute values of and upon the rates of change with temperature of both the a.c. conductivity and the dielectric loss, these parameters being in a major way both temperature and frequency dependent. Thus for instance it is known that a typical soda lime glass can be coupled into a 2 - 5 MHz field at about 1000.degree.C. In this instance the coupling is principally inductive, the relatively high conductivity of this glass being attributed to the sodium ion mobility. In contrast to this a glass composed exclusively of silica and lead oxide, which has a low conductivity even when molten, would require dielectric heating at a much higher frequency typically of several thousand MHz. Inductor and capacitor plate combinations (designed for heating other materials) that would be suitable for this purpose are described in the literature.
During the fining and homogenizing stage of glass preparation the contamination of the melt is relatively small because the source of heat is non-contaminating, because contamination by the crucible can be effectively eliminated by cooling it, and because the melt is contained in a clean environment.
If a preheating stage is required to enable the r.f. field to couple into the melt it is preferable to use a non-contaminating heat source. For many applications, contamination is kept within acceptable bounds when preheating is effected by radiated and conducted heat from a susceptor, typically of graphite, placed over or within the charge of batch materials. Contamination by the graphite can be eliminated by encapsulating it in for instance silicon, silica, or silicon carbide. Silicon may be used on its own as an alternative susceptor material. Other methods of preheating include the use of an r.f. excited plasma flame and also the use of focussed radiation from a high power infra-red lamp.